Climate Destabilization is likely to kick into high gear when we get to the 4°C above preindustrial temperature mark, in about 86 years. At that temperature circumpolar Boreal forest fires and drought in lower latitudes will be returning CO2 too quickly.

Researchers say that extensive modelling shows a four degree temperature rise will be the threshold beyond which CO2 will start to increase more rapidly, as natural carbon 'sinks' of global vegetation become "saturated" and unable to sequester any more CO2 from the Earth's atmosphere.

Carbon sinks are natural systems that drain and store CO2 from the atmosphere, with vegetation providing many of the key sinks that help chemically balance the world - such as the Amazon rainforest and the vast, circumpolar Boreal forest.

As the world continues to warm, consequent events such as Boreal forest fires and mid-latitude droughts will release increasing amounts of carbon into the atmosphere - pushing temperatures ever higher.

Initially, higher atmospheric CO2 will encourage plant growth as more CO2 stimulates photosynthesis, say researchers. But the impact of a warmer world through drought will start to negate this natural balance until it reaches a saturation point.

The modelling shows that global warming of four degrees will result in Earth's vegetation becoming "dominated" by negative impacts - such as 'moisture stress', when plant cells have too little water - on a global scale.

Carbon-filled vegetation 'sinks' will likely become saturated at this point, they say, flat-lining further absorption of atmospheric CO2. Without such major natural CO2 drains, atmospheric carbon will start to increase more rapidly - driving further climate change. [emphasis mine]

Plants are likely to be net carbon emitters by the end of this century, because limited nitrogen and phosphorous will curb growth by 25%.

Taking nitrogen and phosphorus into account brought down the researchers' average prediction of annual global carbon storage by 25% compared ..., the team reports online today in Nature Geoscience. Such a dramatic decline could turn the land from taking up carbon overall to pumping it out by 2100, as the rate of respiration by soil microbes, which exhale carbon dioxide, is predicted to increase in a warmer world. This could mean that Earth gets even hotter as the land starts to amplify human-induced warming rather than slowing it down. [emphasis mine]